1. Field of the Invention
The present invention relates to an AC plane discharge type plasma display panel used as a display device for a display apparatus such as monitor and, more particularly, to an improvement in reliability and display quality of the AC plane discharge type plasma display panel.
2. Background Art
It is a recent trend that in personal computer, etc., not only display monitor of small size and thin type is demanded, but also display image of high brightness and high definition is increasingly required. To satisfy such requirements, several displays using a plasma display panel as a display device have been heretofore developed in various fields of the art, and some of them have already been put into practical use.
FIG. 9 is a partially perspective view showing a structure of a typical AC plane discharge type plasma display panel (hereinafter referred to as AC plane discharge type PDP).
In the drawing, reference numeral 1 indicates a transparent electrode, numeral 2 indicates a bus electrode of a metal for supplying a voltage to the transparent electrode 1, and numeral 11 indicates a fundamental insulating film (hereinafter referred to simply as insulating film) in which light transmission is less lowered. Numeral 3 indicates an even dielectric layer covering the transparent electrode 1 and the bus electrode 2, and numeral 4 indicates a MgO vapor deposition film (hereinafter referred to as cathode film) serving as a cathode at the time of discharge. Numeral 5 indicates a front glass substrate on which the transparent electrode 1, bus electrode 2, dielectric layer 3 and cathode film 4 formed on the insulating film 11 are mounted. These elements form a first substrate section.
Reference numeral 6 indicates a write electrode perpendicularly grade-separating the bus electrode 2, numeral 10 indicates an even graze layer covering the write electrode 6, and numeral 7 indicates a barrier rib for partitioning each individual write electrode 6. Numeral 8 indicates a fluorescent substance formed on the surface of the graze layer 10 and on the wall surface of the barrier rib 7, and subscripts R, G and B means that the fluorescent substances respectively emit fluorescent colors of red, green and blue. Numeral 9 indicates a rear glass substrate on which the mentioned elements 6, 7, 8 and 10 are mounted. These elements form a second substrate section.
Top part of the barrier rib 7 is in contact with the cathode film 4, whereby a discharge space surrounded by the fluorescent substance 8 and the cathode film 4 is formed. This discharge space is filled with a gas mixture of Xe and Ne.
In this construction, as shown in the drawing, a n-th canning line is formed by a pair of transparent electrode 1 and bus electrode 2, i.e., by a pair of electrodes Xn and Yn which sustain discharge.
Each junction at which each scanning line and write electrode 6 are grade-separated forms one discharge cell, and an AC plane discharge type PDP is formed such that a large number of discharge cells are arranged in the form of a matrix.
Generally, as disclosed in the Japanese Laid-Open Patent Publication (unexamined) 95382/1988, a glass substrate used as the front glass substrate 5 or the rear glass substrate 9 in the mentioned AC plane discharge type PDP is a soda lime glass containing about 10 to 20 weight % of sodium oxide, a glass of high distortion point containing less sodium oxide and less influenced by thermal distortion, or others.
In the front glass substrate 5, on the fundamental insulating film 11 of less reduction in light transmittance formed on the surface, a sustain electrode comprising the transparent electrode 1 and the bus electrode 2 is formed by printing process or photolithography mechanical process.
FIG. 10 is a sectional view taken along the line A-Axe2x80x2 in FIG. 9.
With respect to the front glass substrate 5 of the AC plane discharge type PDP, as shown in FIG. 10, a glass substrate formed on the fundamental insulating film 11 of less reduction in light transmittance is generally used.
This is because surface of the glass substrate of the foundation of the transparent electrode 1 is required to be in a condition not containing any sodium oxide, and like structure is popularly adopted in the liquid crystal display (LCD) other than the AC plane discharge type PDP.
The fundamental insulating film 11 performs a function of alkali barrier to prevent that sodium oxide has a negative influence of making unstable the conductivity of the transparent electrode 1 and inhibiting the insulation between the transparent electrodes adjacent each other.
As such a fundamental insulating film 11, there is a known art in which SiO2 film, Si3N4 film, Al2O3 film or the like is formed directly on the glass substrate 5 by sputtering or CVD both being a dry film formation method, as disclosed in the Japanese Laid-Open Patent Publication (unexamined) 95382/1988, for example. Generally, SiO2 film of which formation is easy is popularly adopted in practical use.
In the mentioned construction, the SiO2 film being the fundamental insulating film 11 is a fundamental film of the transparent electrode 1 which is a transparent conductive film of ITO, SnO2, etc., and performs a function of an alkali barrier layer with respect to the front glass substrate 5.
When the layer of the fundamental insulating film 11 is thicker, effect of the alkali barrier is more improved, which is a tradeoff between the effect of alkali barrier and productivity in the formation of SiO2 film.
For example, in case of LCD, when adopting a cheap soda lime glass as a base glass substrate, the fundamental SiO2 film of the transparent electrode performs a necessary and sufficient alkali barrier effect as a result of obtaining a film thickness having values shown in the following Table 1 corresponding to formation method of the SiO2 film.
In this respect, film formation by sputtering is a method for forming a SiO2 film on a substrate by applying a high voltage (several kV) between a cathode to which SiO2 target is attached and an anode opposite thereto in vacuum under an atmosphere of argon from 10xe2x88x922Pa to 100Pa, thereby occurring a glow discharge, and by performing a high frequency sputtering.
Film formation by CVD under normal pressure is a method for forming a SiO2 film by a chemical reaction comprising the steps of heating a substrate, supplying a SiH4 gas to the surface of the substrate, and decomposing and oxidizing the SiH4 on the surface of the substrate.
Both sputtering and CVD belong to a dry film formation method. Further, there is a wet film formation method in which a SiO2 film serving as a alkali barrier film is formed by sol-gel method, as disclosed in the Japanese Laid-Open Patent Publications (unexamined) 303916/1993 and 130307/1995. This film formation by sol-gel method is a method, in which a solution for forming SiO2 containing a catalyst for accelerating hydrolysis reaction and condensation by applying water to silicon alkoxide such as a monomer (C2H5O)4Si of tetraethoxysilane is applied to a substrate composed of a soda lime glass by dipping, roll coating, etc., thereby forming a film, and after drying the film, a SiO2 film is obtained by baking at a temperature of about 500.
Also in the AC plane discharge type PDP, on condition hat the transparent electrodes 1 are not coated with a glass material mainly composed of a lead oxide in the display area as in a DC refresh type PDP, for example, and that there is a less potential difference between the transparent electrodes adjacent each other, the SiO2 film thickness satisfying the mentioned requirements for LCD can perform a sufficient function, even when a soda lime glass containing 10 to 20 weight % of sodium oxide is formed into a base substrate.
When applying such a SiO2 film, however, it was found that there was a problem in the aspect of durability of display quality of the PDP considering an accumulated time of use thereof.
First, when making an evaluation using a SiO2 film of 50 nm in thickness formed by CVD under normal pressure, it was found that life in practical use was in the range of only 500 hours to 1,000 hours.
Then, it was also found that when making an evaluation using a SiO2 film of 100 nm in thickness formed by sol-gel method, there was a durability of the same level.
As a result of examining the cause of such a short life, following problems were acknowledged.
Generally, during the period of writing operation occupying a large portion of time in memory driving, a dc voltage mounting from 100 V to 150 V are applied almost at all times between the n-th sustain electrode Xn and the sustain electrode Yn, and a gap between the sustain electrode Xn and the sustain electrode Yn is so small as to be not larger than 100 xcexcm. Therefore, a strong one-directional electric field acts on the gap portion for most of the time.
As this electric field acts on sodium ion from sodium oxide in the front glass substrate 5, uneven distribution of sodium ion of negative polarity (on the sustain electrode Yn side in this case) becomes remarkable with the passage of time. Thus, sodium component reaching the dielectric layer 3 passing through the SiO2 film is increased.
The unevenly distributed sodium ion reduces the lead oxide in the dielectric layer 3 and precipitates a lead. It is this lead that occurs a migration in which sodium ion is diffused from the base substrate (the front glass substrate 5) and grows from the sustain electrode Yn toward the sustain electrode Xn.
As a result of occurrence of such a migration, even though the applied voltage between the sustain electrode Xn and the sustain electrode Yn is equal, with the passage of time, a distortion arises in the distribution of electric field between the sustain electrode Xn and the sustain electrode Yn. This distortion brings about a large variation in discharge characteristic eventually resulting in disorder in display or lack of stability.
Particularly in the screen of high display rate, temperature of panel is raised, and the mentioned migration remarkably proceeds.
Then, for the purpose of lowering the manufacturing cost, when using a silver of thick film for easy formation of electrode film as a material of the bus electrode 2, color of the bus electrode portion was changed to yellow when watching from the watching side of display of the AC plane discharge type PDP. And in most case, display quality of screen was remarkably deteriorated.
It was acknowledged that this was a following phenomenon. That is, generally, the substrate composed of a soda lime glass formed by floating method contains a metal Sn on the surface. Therefore, when using such a substrate as the front glass substrate 5 serving as the base substrate, with the passage of heat history in the panel formation process, the metal Sn and the silver in the bus electrode are diffused in such a manner as penetrating in direction of thickness of the transparent electrode 1 and the SiO2 film, and react on each other to produce a silver colloid. And this silver colloid produced by the reaction develops the yellow color.
In addition, the substrate composed of a soda lime glass formed by the floating method has a bottom surface containing relatively a large amount of Sn and a top surface containing relatively a small amount of Sn. When using such a bottom surface side as a base, the mentioned color change to yellow becomes considerable finally presenting a brown color. Furthermore, the color change to yellow extends to the light transmission portion having no bus electrode 2, and light transmission characteristic itself of the front glass 5 is deteriorated. Such a substrate cannot be substantially used.
On the other hand, when using the mentioned top surface as a base, there is certainly an advantage that the mentioned color change to yellow is confined only to the bus electrode portion, and the extent of the color change to yellow is relatively a little. But the color change to yellow appears in the form of macroscopically uneven concentration of color on the display screen, which deteriorates the display quality of the picture screen after all.
In addition, it was found that the uneven concentration of color was caused by a film quality of the transparent electrode 1.
Because, when forming the transparent electrode 1 of a SnO2 film by CVD under normal pressure, a significant difference was acknowledged between the following steps (A) and (B).
(A) After forming a SnO2 film by CVD under normal pressure on a SiO2 film on which the transparent electrode 1 and a resist pattern of inverted shape have been formed, the resist pattern was removed, whereby a desired pattern of the transparent electrode 1 was obtained. (lift-off method)
(B) The transparent electrode 1 and a resist pattern of the same shape were formed on a SiO2 film on which a SnO2 film has been formed by CVD under normal pressure. Then, unnecessary portion thereof was removed by chemical etching, and thereafter the resist was removed, whereby a desired pattern of the transparent electrode 1 was obtained. (etching method).
As a result of comparison, it was acknowledged that in the panel obtained by the lift-off method (A), uneven concentration of color appears clearly in most case, while in the panel obtained by the etching method (B), such uneven concentration of color does not appears substantially.
It is presumed that in the lift off method (A), the film is formed under an atmosphere in which at the time of forming the SnO2 film by CVD under normal pressure, the resist is exposed to a high temperature and partially burnt. Therefore, there is a possibility that uneven distribution of combustion components due to gas flow at the time of performing the CVD under normal pressure gives an influence to the film quality of the SnO2 film.
In particular, when sodium in the soda lime glass is partially diffused and reaches the surface where the resist is closely adhered, due to increase of temperature of the glass substrate during the CVD of the SnO2 film under normal pressure, the adhesion of the resist is lost and the resist is peeled off the surface of the substrate. As a result, it is presumed that the resist is burnt more briskly, and the irregularity or unevenness in film quality of the SnO2 film becomes more remarkable.
In this respect, the term xe2x80x9cfilm quality of SnO2 filmxe2x80x9d is defined by following two factors:
(1) a barrier effect on the metal Sn in the base substrate or on the silver in the bus electrode, or
(2) a composition ratio in the film of the components deposited not in the form of SnO2 molecule but in the form of metal Sn in the CVD of the SnO2 film under normal pressure. (A mechanism is supposed in which when the composition ratio of the metal Sn in the SnO2 film is large, this metal Sn comes in contact with the silver of the bus electrode 2 without barrier, whereby the color is changed to yellow.)
The present invention was made to solve the above-discussed problems and has an object of achieving an AC plane discharge type PDP capable of providing a display screen of high definition and high reliability, in which even when a glass containing sodium oxide such as soda lime glass is used as the front glass substrate 5 serving as a base substrate of the first substrate section on the display side of the AC plane discharge type PDP, the color change of the glass substrate to yellow or uneven concentration of color in the yellow after heat history in the step of forming a panel is declined, and even at the time of operation at a high temperature, progress of the migration occurred due to behavior of sodium in the glass substrate is retarded.
Another object of the invention is to achieve an AC plane discharge type PDP of high definition, high reliability and of high productivity.
An AC plane discharge type plasma display panel according to a first invention comprises a first substrate section having a picture screen and a second substrate section arranged opposite to the first substrate section, and in which a desired picture is displayed by a gas discharge in plural discharge cells formed between the first substrate section and the second substrate section,
characterized in that the first substrate section comprises:
a glass substrate containing sodium oxide serving as a base of the first substrate section;
an insulating film being a SiO2 layer having not less than about 100 nm in thickness and formed by dry film formation method on the surface of the second substrate section of the glass substrate;
plural pairs of discharge sustain electrodes each comprising a transparent electrode and a bus electrode, formed on the insulating film, and arranged in parallel with a predetermined distance between one pair and another;
a dielectric layer formed on the insulating film in such a manner as to cover the plural pairs of discharge electrodes; and
a cathode film formed on the dielectric layer.
As a result of above construction, even when the insulating film of SiO2 layer serving as a foundation for forming the transparent electrodes by dry film formation method on the AC plane discharge type PDP, alkali barrier effect can be maintained for a long time. Accordingly, progress of migration can be dull, and an AC plane discharge type PDP of high durability can be achieved.
An AC plane discharge type plasma display panel according to a second invention comprises a first substrate section having a picture screen and a second substrate section arranged opposite to the first substrate section, and in which a desired picture is displayed by a gas discharge in plural discharge cells formed between the first substrate section and the second substrate section,
characterized in that the first substrate section comprises:
a glass substrate containing sodium oxide serving as a base of the first substrate section;
an insulating film being a SiO2 layer having not less than about 200 nm in thickness and formed by wet film formation method on the surface of the second substrate section of the glass substrate;
plural pairs of discharge sustain electrodes each comprising a transparent electrode and a bus electrode, formed on the insulating film, and arranged in parallel with a predetermined distance between one pair and another;
a dielectric layer formed on the insulating film in such a manner as to cover the plural pairs of discharge electrodes; and
a cathode film formed on the dielectric layer.
As a result, even when the insulating film of the SiO2 layer serving as a foundation for forming the transparent electrodes by wet film formation method on the AC plane discharge type PDP, alkali barrier effect can be maintained for a long time. Accordingly, progress of migration can be dull, and an AC plane discharge type PDP of high durability can be achieved.
In the AC plane discharge type plasma display panel according to a third invention or fourth invention, the bus electrode as defined in the mentioned first or second invention is formed using silver of thick film.
As a result, productivity of the bus electrode formation step is improved, and a PDP of high productivity is achieved at a reasonable cost.
Further, as the insulating film of the SiO2 layer can maintain the alkali barrier effect for a long time, progress of migration can be retarded.
The improvement in alkali barrier effect of the insulating film of the mentioned SiO2 layer further brings about an improvement in barrier effect on the diffusion of the metal Sn contained in the mentioned silver or on the surface of the front glass substrate when silver of thick film is used as the bus electrode. Accordingly, as the production of silver colloid can be restrained, the mentioned color change to yellow in the bus electrode portion can be declined.
When the transparent electrode is obtained by the mentioned lift-off method, the function of preventing that sodium of the soda lime glass reaches the surface where the resist is closely adhered during the production of the SnO2 film (i.e., transparent film) is improved. As a result, unevenness in film quality of the SnO2 film (i.e., transparent film) is reduced, and the uneven concentration of color in the color change to yellow of the bus electrode portion can be declined.